Clean up attestation

- Standardize the authenticator data so all formats have equal support
for AAGUID, extensions, etc. This also removes a lot of duplication
across structures.
- Add structure to the definition of attestation formats. Fixes #126.
Fixes #127.
- Simplify the naming of the attestation types to make it easier to
understand
- Clean up mentions of GUID. Fixes #148, fixes #149, fixes #150.
- Clarifies how to use self attestation. Fixes #115.
- More detailed pointers on how to generate a TPM attestation.
- Simplify Android attestation to remove fields that were not really
attested by authenticator and were therefore creating a false sense of
assurance.